Class GregorianCalendar

GregorianCalendar is a concrete subclass of
Calendar and provides the standard calendar system
used by most of the world.

GregorianCalendar is a hybrid calendar that
supports both the Julian and Gregorian calendar systems with the
support of a single discontinuity, which corresponds by default to
the Gregorian date when the Gregorian calendar was instituted
(October 15, 1582 in some countries, later in others). The cutover
date may be changed by the caller by calling setGregorianChange().

Historically, in those countries which adopted the Gregorian calendar first,
October 4, 1582 (Julian) was thus followed by October 15, 1582 (Gregorian). This calendar models
this correctly. Before the Gregorian cutover, GregorianCalendar
implements the Julian calendar. The only difference between the Gregorian
and the Julian calendar is the leap year rule. The Julian calendar specifies
leap years every four years, whereas the Gregorian calendar omits century
years which are not divisible by 400.

GregorianCalendar implements proleptic Gregorian and
Julian calendars. That is, dates are computed by extrapolating the current
rules indefinitely far backward and forward in time. As a result,
GregorianCalendar may be used for all years to generate
meaningful and consistent results. However, dates obtained using
GregorianCalendar are historically accurate only from March 1, 4
AD onward, when modern Julian calendar rules were adopted. Before this date,
leap year rules were applied irregularly, and before 45 BC the Julian
calendar did not even exist.

Prior to the institution of the Gregorian calendar, New Year's Day was
March 25. To avoid confusion, this calendar always uses January 1. A manual
adjustment may be made if desired for dates that are prior to the Gregorian
changeover and which fall between January 1 and March 24.

Values calculated for the WEEK_OF_YEAR field range from 1 to 53. The first week of a
calendar year is the earliest seven day period starting on getFirstDayOfWeek() that contains at
least getMinimalDaysInFirstWeek() days from that year. It thus depends
on the values of getMinimalDaysInFirstWeek(), getFirstDayOfWeek(), and the day of the week of January 1. Weeks
between week 1 of one year and week 1 of the following year
(exclusive) are numbered sequentially from 2 to 52 or 53 (except
for year(s) involved in the Julian-Gregorian transition).

The getFirstDayOfWeek() and getMinimalDaysInFirstWeek() values are initialized using
locale-dependent resources when constructing a GregorianCalendar. The week
determination is compatible with the ISO 8601 standard when getFirstDayOfWeek() is MONDAY and getMinimalDaysInFirstWeek() is 4, which values are used in locales
where the standard is preferred. These values can explicitly be set by
calling setFirstDayOfWeek() and
setMinimalDaysInFirstWeek().

A week year is in sync with a
WEEK_OF_YEAR cycle. All weeks between the first and last
weeks (inclusive) have the same week year value.
Therefore, the first and last days of a week year may have
different calendar year values.

For example, January 1, 1998 is a Thursday. If getFirstDayOfWeek() is MONDAY and getMinimalDaysInFirstWeek() is 4 (ISO 8601 standard compatible
setting), then week 1 of 1998 starts on December 29, 1997, and ends
on January 4, 1998. The week year is 1998 for the last three days
of calendar year 1997. If, however, getFirstDayOfWeek() is
SUNDAY, then week 1 of 1998 starts on January 4, 1998, and
ends on January 10, 1998; the first three days of 1998 then are
part of week 53 of 1997 and their week year is 1997.

Week Of Month

Values calculated for the WEEK_OF_MONTH field range from 0
to 6. Week 1 of a month (the days with WEEK_OF_MONTH =
1) is the earliest set of at least
getMinimalDaysInFirstWeek() contiguous days in that month,
ending on the day before getFirstDayOfWeek(). Unlike
week 1 of a year, week 1 of a month may be shorter than 7 days, need
not start on getFirstDayOfWeek(), and will not include days of
the previous month. Days of a month before week 1 have a
WEEK_OF_MONTH of 0.

For example, if getFirstDayOfWeek() is SUNDAY
and getMinimalDaysInFirstWeek() is 4, then the first week of
January 1998 is Sunday, January 4 through Saturday, January 10. These days
have a WEEK_OF_MONTH of 1. Thursday, January 1 through
Saturday, January 3 have a WEEK_OF_MONTH of 0. If
getMinimalDaysInFirstWeek() is changed to 3, then January 1
through January 3 have a WEEK_OF_MONTH of 1.

Default Fields Values

The clear method sets calendar field(s)
undefined. GregorianCalendar uses the following
default value for each calendar field if its value is undefined.

Method Detail

setGregorianChange

Sets the GregorianCalendar change date. This is the point when the switch
from Julian dates to Gregorian dates occurred. Default is October 15,
1582 (Gregorian). Previous to this, dates will be in the Julian calendar.

To obtain a pure Julian calendar, set the change date to
Date(Long.MAX_VALUE). To obtain a pure Gregorian calendar,
set the change date to Date(Long.MIN_VALUE).

Parameters:

date - the given Gregorian cutover date.

getGregorianChange

Gets the Gregorian Calendar change date. This is the point when the
switch from Julian dates to Gregorian dates occurred. Default is
October 15, 1582 (Gregorian). Previous to this, dates will be in the Julian
calendar.

Returns:

the Gregorian cutover date for this GregorianCalendar object.

isLeapYear

public boolean isLeapYear(int year)

Determines if the given year is a leap year. Returns true if
the given year is a leap year. To specify BC year numbers,
1 - year number must be given. For example, year BC 4 is
specified as -3.

Parameters:

year - the given year.

Returns:

true if the given year is a leap year; false otherwise.

equals

Compares this GregorianCalendar to the specified
Object. The result is true if and
only if the argument is a GregorianCalendar object
that represents the same time value (millisecond offset from
the Epoch) under the same
Calendar parameters and Gregorian change date as
this object.

add

public void add(int field,
int amount)

Adds the specified (signed) amount of time to the given calendar field,
based on the calendar's rules.

Add rule 1. The value of field
after the call minus the value of field before the
call is amount, modulo any overflow that has occurred in
field. Overflow occurs when a field value exceeds its
range and, as a result, the next larger field is incremented or
decremented and the field value is adjusted back into its range.

Add rule 2. If a smaller field is expected to be
invariant, but it is impossible for it to be equal to its
prior value because of changes in its minimum or maximum after
field is changed, then its value is adjusted to be as close
as possible to its expected value. A smaller field represents a
smaller unit of time. HOUR is a smaller field than
DAY_OF_MONTH. No adjustment is made to smaller fields
that are not expected to be invariant. The calendar system
determines what fields are expected to be invariant.

roll

public void roll(int field,
boolean up)

Adds or subtracts (up/down) a single unit of time on the given time
field without changing larger fields.

Example: Consider a GregorianCalendar
originally set to December 31, 1999. Calling roll(Calendar.MONTH, true)
sets the calendar to January 31, 1999. The YEAR field is unchanged
because it is a larger field than MONTH.

roll

public void roll(int field,
int amount)

Adds a signed amount to the specified calendar field without changing larger fields.
A negative roll amount means to subtract from field without changing
larger fields. If the specified amount is 0, this method performs nothing.

This method calls Calendar.complete() before adding the
amount so that all the calendar fields are normalized. If there
is any calendar field having an out-of-range value in non-lenient mode, then an
IllegalArgumentException is thrown.

Example: Consider a GregorianCalendar
originally set to August 31, 1999. Calling roll(Calendar.MONTH,
8) sets the calendar to April 30, 1999. Using a
GregorianCalendar, the DAY_OF_MONTH field cannot
be 31 in the month April. DAY_OF_MONTH is set to the closest possible
value, 30. The YEAR field maintains the value of 1999 because it
is a larger field than MONTH.

Example: Consider a GregorianCalendar
originally set to Sunday June 6, 1999. Calling
roll(Calendar.WEEK_OF_MONTH, -1) sets the calendar to
Tuesday June 1, 1999, whereas calling
add(Calendar.WEEK_OF_MONTH, -1) sets the calendar to
Sunday May 30, 1999. This is because the roll rule imposes an
additional constraint: The MONTH must not change when the
WEEK_OF_MONTH is rolled. Taken together with add rule 1,
the resultant date must be between Tuesday June 1 and Saturday June
5. According to add rule 2, the DAY_OF_WEEK, an invariant
when changing the WEEK_OF_MONTH, is set to Tuesday, the
closest possible value to Sunday (where Sunday is the first day of the
week).

For example, if the Gregorian change date is January 10,
1970 and the date of this GregorianCalendar is
January 20, 1970, the actual minimum value of the
DAY_OF_MONTH field is 10 because the previous date
of January 10, 1970 is December 27, 1996 (in the Julian
calendar). Therefore, December 28, 1969 to January 9, 1970
don't exist.

getActualMaximum

public int getActualMaximum(int field)

Returns the maximum value that this calendar field could have,
taking into consideration the given time value and the current
values of the
getFirstDayOfWeek,
getMinimalDaysInFirstWeek,
getGregorianChange and
getTimeZone methods.
For example, if the date of this instance is February 1, 2004,
the actual maximum value of the DAY_OF_MONTH field
is 29 because 2004 is a leap year, and if the date of this
instance is February 1, 2005, it's 28.

This method calculates the maximum value of WEEK_OF_YEAR based on the YEAR (calendar year) value, not the week year. Call getWeeksInWeekYear() to get the maximum value of WEEK_OF_YEAR in the week year of this GregorianCalendar.

getWeekYear

public int getWeekYear()

Returns the week year represented by this
GregorianCalendar. The dates in the weeks between 1 and the
maximum week number of the week year have the same week year value
that may be one year before or after the YEAR
(calendar year) value.

Note that the numeric day-of-week representation differs from
the ISO 8601 standard, and that the weekOfYear
numbering is compatible with the standard when getFirstDayOfWeek() is MONDAY and getMinimalDaysInFirstWeek() is 4.

Unlike the set method, all of the calendar fields
and the instant of time value are calculated upon return.

If weekOfYear is out of the valid week-of-year
range in weekYear, the weekYear
and weekOfYear values are adjusted in lenient
mode, or an IllegalArgumentException is thrown in
non-lenient mode.